Feedforward receiver and method for reducing inter-symbol interference by using joint soft values

Abstract

A feedforward receiver and method are described herein that address inter-symbol interference in received symbols by using an enhanced equalizer to generate joint soft values (joint information of a previous modem bit x′ and a modem bit x) and an enhanced decoder which uses the joint soft values and side information (bias about the previous modem bit x′) to output a more reliable information bit x.

1. A feedforward receiver for reducing inter-symbol interference in a received symbol, said receiver comprising:
 an equalizer that determines and outputs a joint soft value which includes joint information of a modem bit x and a previous modem bit x′, wherein determining the joint soft value comprises computing the equation:
     E ″( x′,x )= F   k−2 ( j″   x′,x )= f   k−1 ( j″   x′,x   ,j′   x )= f   k ( j′   x ,φ),
 
 wherein E″(x′,x) defines a cumulative metric candidate,
 F k−2 (j″ x′,x ) defines a cumulative metric for index k−2, 
 k defines a time index, 
 f k−1 (j″ x′,x ,j′ x ) defines a branch metric at index k−1, 
 f k−1 (j′ x ,φ) defines a branch metric at index k; 
 
 wherein x comprises a modem bit labeling a branch from stage k−1 to
 stage k, 
 φ comprises a value corresponding to a best state at index k, 
 j′ x  comprises a value corresponding to a state at index k−1, 
 j′ x′,x  comprises a value corresponding to a state at index k−2; 
 
 
 a fusion function that receives the joint soft value and outputs an input soft value for the previous modem bit x′; 
 a decoder that receives the input soft value for the previous modem bit x′ and outputs an output soft value for the previous modem bit x′; and 
 a modification unit that uses the input soft value for the previous modem bit x′ and the output soft value for the previous modem bit x′ to generate side information which is in a form of a bias about the previous modem bit x′; 
 wherein said fusion function receives the side information and processes the side information and the joint soft value to generate an improved single soft value for modem bit x. 
 
     
     
       2. The feedforward receiver of  claim 1 , further comprising a deinterleaver located between the equalizer and the fusion function where the deinterleaver changes indices in the modem bits x and x′. 
     
     
       3. The feedforward receiver of  claim 1 , wherein the modification unit is a subtraction unit that subtracts the input soft value for the previous modem bit x′ from the output soft value for the previous modem bit x′ to generate side information which is in the form of the bias about the previous modem bit x′. 
     
     
       4. The feedforward receiver of  claim 1 , wherein the equalizer uses a cheap Soft Output Viterbi Algorithm (SOVA) technique to generate the joint soft value. 
     
     
       5. The feedforward receiver of  claim 1 , wherein the equalizer uses a SOVA technique to generate the joint soft value. 
     
     
       6. The feedforward receiver of  claim 1 , wherein the previous modem bit x′ and the modem bit x belong to a same codeword. 
     
     
       7. The feedforward receiver of  claim 1 , wherein the previous modem bit x′ belongs to a first codeword and the modem bit x belongs to a second codeword. 
     
     
       8. The feedforward receiver of  claim 1 , wherein the previous modem bit x′ belongs to a first codeword and the modem bit x is uncoded. 
     
     
       9. The feedforward receiver of  claim 1 , wherein the equalizer generates and forwards the input soft value for the previous modem bit x′ to the fusion function. 
     
     
       10. The feedforward receiver of  claim 1 , wherein the equalizer generates the joint soft value based on binary modulation, M-ary PSK modulation, or M-ary QAM modulation, wherein M is a positive integer. 
     
     
       11. The feedforward receiver of  claim 1 , wherein the decoder is a Maximum A Posteriori (MAP) decoder which uses a forward recursion, a backward recursion and a combining step to produce the output soft value for the previous modem bit x′. 
     
     
       12. The feedforward receiver of  claim 1 , wherein the decoder uses a punctured convolutional code to generate the output soft value for the previous modem bit x′. 
     
     
       13. The feedforward receiver of  claim 1 , wherein the decoder uses a turbo code to generate the output soft value for the previous modem bit x′. 
     
     
       14. The feedforward receiver of  claim 1 , wherein the decoder uses a block code to generate the output soft value for the previous modem bit x′. 
     
     
       15. The feedforward receiver of  claim 1 , wherein the equalizer uses the modem bit x and the previous modem bit x′ which are from neighboring stages to generate the joint soft value. 
     
     
       16. The feedforward receiver of  claim 1 , wherein the equalizer uses the modem bit x and the previous modem bit x′ which are from non-neighboring stages to generate the joint soft value. 
     
     
       17. The feedforward receiver of  claim 1 , wherein the modification unit generates side information which is in a form of a bias about the modem bit x instead of the previous modem bit x′, and the fusion function receives the side information and processes the side information and the joint soft value to generate an improved single soft value for the previous modem bit x′ instead of the modem bit x. 
     
     
       18. A method for reducing inter-symbol interference in a received symbol, said method comprising the steps of:
 generating a joint soft value which includes joint information of a modem bit x and a previous modem bit x′ using an equalizer, wherein generating the joint soft value comprises computing the equation:
     E ″( x′,x )= F   k−2 ( j″   x′,x )= f   k=1 ( j″   x′,x , j′   x )= f   k ( j′   x ,φ),
 
 wherein E″(x′,x) defines a cumulative metric candidate,
 F k−2 (j″ x′,x ) defines a cumulative metric for index k−2, 
 k defines a time index, 
 f k−1 (j″ x′,x ,j′ x ) defines a branch metric at index k−1, 
 f k (j′ x ,φ) defines a branch metric at index k; 
 
 wherein x comprises a modem bit labeling a branch from stage k−1 to
 stage k, 
 φ comprises a value corresponding to a best state at index k, 
 j′ x  comprises a value corresponding to a state at index k−1, 
 j″ x′,x  comprises a value corresponding to a state at index k−2; and 
 
 
 using the joint soft value and side information which is in a form of a bias about the previous modem bit x′ or the modem bit x to respectively generate a single soft value for the modem bit x or the previous modem bit x′. 
 
     
     
       19. The method of  claim 18 , wherein the equalizer is not used again during the using step in which a decoder is used to generate the single soft value for the modem bit x or the previous modem bit x′. 
     
     
       20. The method of  claim 18 , wherein the generating step further includes using a cheap SOVA technique or a SOVA technique to generate the joint soft value. 
     
     
       21. The method of  claim 18 , wherein the generating step further includes generating the joint soft value based on binary modulation, M-ary PSK modulation, or M-ary QAM modulation. 
     
     
       22. The method of  claim 18 , wherein the generating step uses the modem bit x and the previous modem bit x′ which are from neighboring stage. 
     
     
       23. The method of  claim 18 , wherein the generating step uses the modem bit x and the previous modem bit x′ which are from non-neighboring stages. 
     
     
       24. The method of  claim 18 , wherein the joint soft value includes joint information of multiple modem bits ( . . . , x″,x′,x), with side information for each of previous modem bits ( . . . , x″,x′) being fused with the multiple modem bit joint soft value to benefit the last modem bit x .